Thank you for your help! You made me look at my setup again; I will write more later, but I now have a widget on a UI page that successfully sends the arm or disarm command to my AlarmDecoder. Short story, I had gotten hung up on the fact that I expected to be able to enter text when the UI showed the item in the model view, and it only shows NULL and is not editable. Did not need to change the thing or item or link, except I need to add YOUR code, to expire the command string, to the item.
As followup, under FreeBSD the configuration files are located at /var/db/openhab/userdata/jsondb. This directory holds the files generated by the UI configuration method. To expire an item, configuring the item using the UI, one must add metadata to the item, specifically, in the case of the AlarmDecoder binding, to the Alarm Keypad Alarm Deocder Keypad Command item (or whatever you have named it). Please see screenshot below, from the Model view:
Metadata is held in the file org.openhab.core.items.Metadata.json. The relevent section of the metadata file looks like this:
"expire:AlarmKeypadAlarmDecoder_KeypadCommand": {
"class": "org.openhab.core.items.Metadata",
"value": {
"key": {
"segments": [
"expire",
"AlarmKeypadAlarmDecoder_KeypadCommand"
],
"uid": "expire:AlarmKeypadAlarmDecoder_KeypadCommand"
},
"value": "0h0m2s,NULL",
"configuration": {}
}
},